The present invention relates to a method and apparatus for washing and drying a substrate such as a semiconductor wafer or a substrate for a liquid crystal display (LCD) device, in which a washing solution is applied to the substrate which is kept rotated on its own axis for the washing purpose.
In the manufacture of a semiconductor device, a washing treatment is applied in general to a substrate for removing particles attached to the surface thereof and a spontaneous oxide film formed on the substrate surface as a result of contact of the substrate surface with the air. A one-by-one type washing method in which substrates are washed one by one using a spin type washing device is known to the art as a method of washing a substrate. In the spin type washing system, a chemical solution, e.g., hydrofluoric acid solution, is applied to the surface of a substrate while rotating the substrate on its own axis, followed by washing the substrate with a washing solution, e.g., pure water. Further, the substrate is rotated at a high speed so as to centrifugally remove the liquid material from the substrate.
FIGS. 1A to 1D collectively show a conventional washing-drying method. In the first step, a wafer W is held by a spin chuck 1. Under this condition, a hydrofluoric acid solution A is applied from a nozzle 2 onto the wafer W while rotating the wafer W, as shown in FIG. 1A. Then, a pure water B is applied from a nozzle 3 to the wafer W while rotating the wafer W, as shown in FIG. 1B. In this step, the pure water B partly remains in an irregular surface region of the wafer W to form water droplets Ba, as shown in FIG. 1C. After drying, water marks 4 are formed in the portions where the water droplets Ba were formed previously, as shown in FIG. 1D. It should be noted that traces of foreign substances contained in the water droplets Ba are precipitated on the substrate surface and left on the substrate surface after drying to form "water mark" noted above. In general, the water mark represents a thin film-like foreign substance consisting of silicon-based oxides. These water marks are counted, and the number of counts provides an index denoting the grade of drying state of the substrate.
The mechanism of the water mark formation is considered to be as follows. Specifically, the volume of water B attached to the wafer W is gradually decreased as a result of evaporation to form finally a spherical water droplet Ba. The spherical water droplet Ba thus formed reacts with oxygen and silicon to form H.sub.2 SiO.sub.3 which is to be precipitated. Alternatively, traces of silica (SiO.sub.2) contained in the pure water B is precipitated, leading to formation of the water mark 4. Particularly, where a hydrofluoric acid solution is used as a washing solution, the spontaneous oxide film of SiO2 covering the surface of the wafer W is removed so as to expose the silicon wafer itself to the outside, with the result that the above-noted reaction leading to the water mark formation is likely to take place easily.
FIGS. 11 and 12 show more specifically the mechanism of the water mark formation. FIG. 11 shows that a hydrophilic thin film F1 formed on a silicon wafer W is patterned, followed by forming a hydrophobic thin film F2 such as a polycrystalline silicon film on the thin film F1. Likewise, FIG. 12 shows that a hydrophilic thin film F3 formed on a silicon wafer W is patterned, followed by forming a hydrophobic thin film F4 on the thin film F3. In these cases, water droplets Ba tend to remain in the concavities (contact holes) 5, 5A of the hydrophobic thin films F2, F4. Since it is difficult to remove these water droplets Ba remaining in the concavities 5, 5A, a large number of water marks 4 are formed on the surface of the silicon wafer W as shown in, for example, FIGS. 15B and 15C.